3,170 Works

Continuous Navigation of an Inland Vessel with a Synthetic GNSS Antenna

Tobias Kersten, Le Ren & Steffen Schön
This paper describes a concept to obtain a continuous navigation and position solution of inland vessels based on a multi-GNSS antenna system. Also known as, “Virtual Receiver” we utilize this approach as an alternative method with respect to a common dead reckoning procedure. Such an approach strengthens the geometry of visible GNSS satellites immediatelyby up to 50%. At the same time, dilution of precision values improve by up to 40%. Hence, continuous navigation solution under...

A Virtual Receiver Concept for Continuous GNSS based Navigation of Inland Vessels

Tobias Kersten, Le Ren & Steffen Schön
Efficient and economic guidance of inland vessels relies on a continuous, available, reliable and precise GNSS navigation solution. Hence, below other side effects, this is especially critical when passing beneath bridges or similar infrastructures that cross waterways. They have two effects: distortion (reflection, diffraction and interruption) of the incoming GNSS signal by the individual bridge structure on the one hand and along with that, the affected ambiguity resolution for carrier phase observation on the other...

Netzoptimierung durch Wirk- und Blindleistungsredispatch auf Basis von konvexifizierten quadratischen Näherungen der Netzgleichungen

Thomas Leveringhaus
Die Energiewende, der Kernenergieausstieg, eine zunehmende Elektrifizierung energetischer Anwendungen, ein zunehmender internationaler Stromhandel, ein unzureichender Netzausbau und ein zunehmender regulatorischer Effizienzdruck bewirken einen Betrieb des elektrischen Energieversorgungsnetzes an seinen Betriebsgrenzen beziehungsweise erfordern in zunehmenden Umfang Maßnahmen, um diese einzuhalten. Da die Betriebsgrenzen und Maßnahmen miteinander in Wechselwirkung stehen, ergibt sich ein herausforderndes Optimierungsproblem. Diese Arbeit enthält Weiterentwicklungen der Modellierung und Lösung eines Optimierungsproblems, welches die Minimierung der Kosten für Netzverluste und die Minimierung der Kosten...

Stoichiometry of the photocatalytic fuel production by the reformation of aqueous acetic acid

Saher Hamid
In recent years, photocatalytic reaction systems gained much attention to address both energy and environmental issues. One of these photocatalytic reactions involve the conversion of aqueous organic pollutants resulting from industrial waste into value added fuels (i.e., H2, and hydrocarbons) by means of solar-light. However, less understanding about reaction mechanism limits the practical realization of such vital process. Therefore, fundamental research in this field is very important to improve the photocatalytic efficiency of these systems....

Proceedings of the 4th Summer School on Video Compression and Processing (SVCP) 2018

Hossein Bakhshi Golestani, Johannes Bauer, Johannes Erfurt, Kristian Fischer, Alexander Gehlert, Nils Genser, Simon Grosche, Felix Kuhnke, Thorsten Laude, Holger Meuel, Marco Munderloh, Andreas Spruck & Jan Voges
Proceedings of the 4th Summer School on Video Compression and Processing (SVCP) 2018

Femtosecond Optoinjection of Intact Tobacco BY-2 Cells Using a Reconfigurable Photoporation Platform

Claire A. Mitchell, Stefan Kalies, Tomas Cizmar, Alexander Heisterkamp, Lesley Torrance, Alison G. Roberts, Frank J. Gunn-Moore & Kishan Dholakia
A tightly-focused ultrashort pulsed laser beam incident upon a cell membrane has previously been shown to transiently increase cell membrane permeability while maintaining the viability of the cell, a technique known as photoporation. This permeability can be used to aid the passage of membrane-impermeable biologically-relevant substances such as dyes, proteins and nucleic acids into the cell. Ultrashort-pulsed lasers have proven to be indispensable for photoporating mammalian cells but they have rarely been applied to plant...

Optimal directed searches for continuous gravitational waves

Jing Ming
Kurzfassung Am 14. September 2015 hat LIGO, das Laser Interferometer Gravitational-Wave Observatory, zum ersten Mal Gravitationswellen (GW) nachgewiesen. GW sind eine der bedeutendsten Vorhersagen von Einsteins Allgemeiner Relativitätstheorie. Dieses erste GW-Ereignis, GW150914, war die katastrophale Verschmelzung zweier Schwarzer Löcher stellarer Masse, aus den Tiefen des Universums. Seitdem steht ein neues Fenster auf das Universum offen. Seit GW150914 wurden GW mehrerer anderer binärer Schwarzer Löcher beobachtet, während kontinuierliche GW (CW, für Continuous gravitational Waves) von schnell...

Enabling second harmonic generation as a contrast mechanism for optical projection tomography (OPT) and scanning laser optical tomography (SLOT)

Lena Nolte, Georgios C. Antonopoulos, Lisa Rämisch, Alexander Heisterkamp, Tammo Ripken & Heiko Meyer
Volumetric imaging of connective tissue provides insights into the structure of biological tissue. Second harmonic generation (SHG) microscopy has become a standard method to image collagen rich tissue like skin or cornea. Due to the non-centrosymmetric architecture, no additional label is needed and tissue can be visualized noninvasively. Thus, SHG microscopy enables the investigation of collagen associated diseases, providing high resolution images and a field of view of several hundreds of µm. However, the in...

Analysis of poration-induced changes in cells from laser-activated plasmonic substrates

Nabiha Saklayen, Stefan Kalies, Marinna Madrid, Valeria Nuzzo, Marinus Huber, Weilu Shen, Jasmine Sinanan-Singh, Dag Heinemann, Alexander Heisterkamp & Eric Mazur
Laser-exposed plasmonic substrates permeabilize the plasma membrane of cells when in close contact to deliver cell-impermeable cargo. While studies have determined the cargo delivery efficiency and viability of laser-exposed plasmonic substrates, morphological changes in a cell have not been quantified. We porated myoblast C2C12 cells on a plasmonic pyramid array using a 532-nm laser with 850-ps pulse length and time-lapse fluorescence imaging to quantify cellular changes. We obtain a poration efficiency of 80%, viability of...

Elements of a dielectric laser accelerator

Joshua McNeur, Martin Kozák, Norbert Schönenberger, Kenneth J. Leedle, Huiyang Deng, Andrew Ceballos, Heinar Hoogland, Axel Ruehl, Ingmar Hartl, Ronald Holzwarth, Olav Solgaard, James S. Harris, Robert L. Byer & Peter Hommelhoff
We experimentally demonstrate several physical concepts necessary for the future development of dielectric laser accelerators—photonic elements that utilize the inelastic interaction between electrons and the optical near fields of laser-illuminated periodic nanostructures. To build a fully photonic accelerator, concatenation of elements, large energy gains, and beam steering elements are required. Staged acceleration is shown using two spatio-temporally separated interaction regions. Further, a chirped silicon grating is used to overcome the velocity dephasing of subrelativistic electrons...

Picosecond near-to-mid-infrared absorption of pulse-injected small polarons in magnesium doped lithium niobate

Felix Freytag, Philip Booker, Gabor Corradi, Simon Messerschmidt, Andreas Krampf & Mirco Imlau
Femtosecond-pulse-induced (Epump = 2:5 eV) picosecond infrared absorption is studied in the spectral region between 0.30 eV and 1.05 eV in LiNbO3:Mg. We find a noninstantaneous mid-infrared absorption peak in the time domain up to 1 ps and a broad-band, long-lived absorption (maximum at 0.85 eV, width ≈ 0:5 eV), for t > 1 ps. The modelling succeeds by considering small Nb4+ Nb electron polaron formation along the sequence: (i) twophoton injection of hot electron-hole...

Multifocal diffractive lens generating several fixed foci at different design wavelengths

Leonid L. Doskolovich, Evgeni A. Bezus, Andrey A. Morozov, Vladimir Osipov, James S. Wolffsohn & Boris Chichkov
We propose a method for designing multifocal diffractive lenses generating prescribed sets of foci with fixed positions at several different wavelengths. The method is based on minimizing the difference between the complex amplitudes of the beams generated by the lens microrelief at the design wavelengths, and the functions of the complex transmission of multifocal lenses calculated for these wavelengths. As an example, a zone plate generating three fixed foci at three different wavelengths was designed,...

Resource allocation in a multi-color DS-OCDMA VLC cellular architecture

Marwan Hammouda, Anna Maria Vegni, Jürgen Peissig & Mauro Biagi
In this paper we present two resource allocations techniques in a visible light communication network with overlapping coverage areas due to the use of access points. Particularly, the first approach exploits the rate maximization criteria, and then aims at maximizing the network rate under constraints on minimum and maximum rates, while the other procedure focuses on achieving fairness in the rate of each user accessing the network. The proposed system relays on optical code division...

High repetition rate, J-level, CPA-free ultrashort pulse multipass amplifier based on Ho:YLF

Moritz Hinkelmann, Dieter Wandt, Uwe Morgner, Jörg Neumann & Dietmar Kracht
We report on CPA-free multipass amplification of ps-pulses in Holmium-doped yttrium lithium fluoride (Ho:YLF) crystals up to µJ pulse-energy-covering repetition rates from 10 kHz up to 500 kHz. The seed pulses at a wavelength of 2.05 µm are provided by a Ho-based all-fiber system consisting of a soliton oscillator and a subsequent pre-amplifier followed by a free-space AOM as pulse-picker. Considering the achieved pulse peak power at MW-level, this system is a powerful tool for...

Line of sight calibration for the laser ranging interferometer on-board the GRACE follow-On mission: On-ground experimental validation

Alexander Koch, Josep Sanjuan, Martin Gohlke, Christoph Mahrdt, Nils Christopher Brause, Claus Braxmaier & Gerhard Heinzel
The laser ranging interferometer (LRI) on board of the GRACE follow-on spacecraft, launched in May 2018, is the first laser interferometer to perform an inter-satellite range measurement. It is designed for ranging noise levels of 80 nm Hz−1/2 for frequencies above 20 mHz, i.e., about a ten-fold improvement with respect to the GRACE follow-on main microwave ranging instrument. One of the most critical steps during the commissioning phase of the instrument is the so-called initial...

Predictor-corrector framework for the sequential assembly of optical systems based on wavefront sensing

Christopher Schindlbeck, Christian Pape & Eduard Reithmeier
Alignment of optical components is crucial for the assembly of optical systems to ensure their full functionality. In this paper we present a novel predictor-corrector framework for the sequential assembly of serial optical systems. Therein, we use a hybrid optical simulation model that comprises virtual and identified component positions. The hybrid model is constantly adapted throughout the assembly process with the help of nonlinear identification techniques and wavefront measurements. This enables prediction of the future...

Enhancement of the damage resistance of ultra-fast optics by novel design approaches

Thomas Willemsen, Marco Jupé, Mark Gyamfi, Sebastian Schlichting & Detlev Ristau
Dielectric components are essential for laser applications. Chirped mirrors are applied to compress the temporal pulse broadening crucial in the femtosecond regime. However, the design sensitivity and the electric field distribution of chirped mirrors is complex often resulting in low laser induced damage resistances. An approach is presented to increase the damage resistance of pulse compressing mirrors up to 190% in the NIR spectral range. Layers with critical high field intensity of a binary mirror...

Switching between one and two dimensions: Conductivity of Pb-induced chain structures on Si(557)

Christoph Tegenkamp, Z. Kallassy, Herbert Pfnür, H.-L. Günter, V. Zielasek & M. Henzler
We show in a combined study of four-point conductance measurement and tunneling microscopy that surface state conductance induced by one monolayer of Pb on Si(557) can be quasi one dimensional with conductivity values close to typical three-dimensional metals. At a critical temperature of Tc=78K, associated with an order-disorder phase transition and a tenfold superperiodicity along the Pb chains, the system switches from low to high conductance anisotropy, with a semiconductor-insulator transition in the direction perpendicular...

Roughness and stability of silicon on insulator surfaces

M. Czubanowski, Christoph Tegenkamp, W. Ernst & Herbert Pfnür
The feasibility of low temperature processes (below 800°C), to obtain atomically clean and smooth surfaces on (100) oriented silicon on insulator material (SOI) with negligible variation of the top Si film thickness was tested. These processes were characterized by using low-energy electron diffraction (LEED) and atomic force microscopy (AFM) supplemented by conductivity measurements. The evaporation of Si at 750°C at a flux of 0.15 ML/min, the most promising method for obtaining atomically smooth and continuous...

Spin-resolved band structure of a densely packed Pb monolayer on Si(111)

C. Brand, S. Muff, M. Fanciulli, Herbert Pfnür, M.C. Tringides, J.H. Dil & Christoph Tegenkamp
Monolayer structures of Pb on Si(111) attracted recently considerable interest as superconductivity was found in these truly two-dimensional (2D) structures. In this study, we analyzed the electronic surface band structure of the so-called striped incommensurate Pb phase with 43 ML coverage by means of spin-resolved photoemission spectroscopy. Our results fully agree with density functional theory calculations done by Ren et al. [Phys. Rev. B 94, 075436 (2016)1098-012110.1103/PhysRevB.94.075436]. We observe a local Zeeman-type splitting of a...

Tank based unified torque/impedance control for a pneumatically actuated antagonistic robot joint

Alexander Tödtheide, Erfan Shahriari & Sami Haddadin
In this paper the concept of a unified torque/impedance controller is applied to a pneumatically actuated, antagonistic robot joint. The investigated control algorithm consists of a cascaded structure in which the outer torque/impedance controller commands a desired torque to two cylinder-based force controllers. The torque-/impedance controller is equipped with a virtual tank that ensures passivity of the control loops above force level. Additionally, a shaping function provides a continuous transition to impedance control in case...

Thiol and thiolate bond formation of ferrocene-1,1-dithiol to a Ag(111) surface

J. Meyer, T. Bredow, Christoph Tegenkamp & Herbert Pfnür
Using density functional calculations, we show that the adsorption of ferrocene dithiol on the Ag(111) surface is remarkably flexible, i.e., a large number of different configurations have binding energies that differ by less than 0.1 eV per molecule. The thiolate bond is slightly favored over the thiol bond (by less than 0.1 eV) but may not be formed due to considerable activation barriers. Electronically, we found that the thiolate bound molecule is conducting, whereas thiol...

Entanglement and extreme spin squeezing of unpolarized states

G. Vitagliano, Iagoba Apellaniz, M. Kleinmann, Bernd Lücke, Carsten Klempt & Geza Toth
We present criteria to detect the depth of entanglement in macroscopic ensembles of spin-j particles using the variance and second moments of the collective spin components. The class of states detected goes beyond traditional spin-squeezed states by including Dicke states and other unpolarized states. The criteria derived are easy to evaluate numerically even for systems of very many particles and outperform past approaches, especially in practical situations where noise is present. We also derive analytic...

Formation of surface color centers at differently coordinated sites: MgO/Ag(1,1,19)

J. Kramer, Christoph Tegenkamp & Herbert Pfnür
In order to find criteria for discrimination between surface color centers at terraces and steps, and bulk color centers, the characteristic losses of these centers have been investigated by electron energy loss spectroscopy on an epitaxial MgO film grown on a Ag (1,1,19) surface. This film contains a significantly higher step density compared with a film grown on Ag(100). The generation of four distinct losses at 2.0, 2.4, 2.8, and 3.4 eV and a broad...

Über Photomagnetismus und Photochromie II

G. Kortüm, G. Littmann & W. Theilacker
Im I. Teil dieser Arbeit1 wurden Untersuchungen über den photomagnetischen Effekt (Molekeln im Triplettzustand) beschrieben. Der II. Teil zeigt, daß die photochrome Modifikation der Derivate des Dehydrodianthrons vermutlich kein Triplettzustanddes Moleküls, sondern höchstwahrscheinlich das Doppelradikal ist, wie schon die optischen Untersuchungen vermuten ließen.

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  • Leibniz Universität Hannover